Heat and Mass Transfer

Effects of EHD on the flow and heat transfer characteristics in a rectangular corrugated channel Abstract The electrohydrodynamic (EHD) effects on the flow field and forced convection heat transfer in a rectangular corrugated channel is numerically investigated in the present study. The numerical results are validated against experimental data for the smooth channel. The simulation results qualitatively agree with the experimental measurements. It is observed that in the presence of EHD in the rectangular corrugated channel, the thermal enhancement index at higher aspect ratios becomes better rather than the lower ones. Also, the application of the corona voltage, yields a stronger recirculation zone and causes a greater increment in the heat transfer rate.

Numerical investigation of flow and heat transfer in enhanced tube with slot dimples Abstract In this paper, a novel enhanced tube with slot dimples aiming to improve heat transfer have been put forward. The flow and heat transfer characteristics of the enhanced tube with slot dimples (ETSD) were numerically analysed and compared with spherical/elliptical dimples. The distributions of temperature, velocity, pressure, Nusselt number and streamlines were carried out to describe the mechanism of heat transfer and fluid flow. Additionally, the effects of dimple depth, length and axis ratios on turbulent fluid flow and heat transfer performances were also being studied in details. It is found that the enhanced tube with slot dimples have an advantage for augmented heat transfer rate compared with the spherical/elliptical dimple tube due to the slot dimples generated greater swirling flow, better fluid mixing, and greater flow blockage. In addition, the slot dimples destroyed the boundary layer, intense flow mixing and formed periodic impinge flows, thus significant improved of thermal–hydraulic performance. The Nu/Nu0 and f/f0 for ETSD increases with the increasing of dimples depth, and decreases with increasing of pitch. The ETSD with D = 1.5 mm, P = 30 mm, R = 2.33 and Re = 5000 provided the largest PEC value about 2.02 in all the case.

New approach to achieving smaller bubbles with various microwave irradiation modes Abstract Previous studies have measured the sizes of bubbles in suspension during microwave irradiation to clarify the mechanism of bubble formation. Those results indicate that the particle number density and the dielectric constants of the particles and solvent are the most important factors determining bubble size during irradiation. The aim of this study is to determine whether bubble nucleation can be controlled by changing the microwave mode between continuous irradiation and the two-stage irradiation proposed in this study. The first irradiation of higher power rapidly accelerates bubble nucleation, whereupon the bubbles grow more slowly during the second irradiation of lower power. Because the absorbed microwave energy is distributed to the liquid–air interface of each highly suspended small bubble produced by the first irradiation, the absorbed energy per bubble decreases during the second irradiation. Finally, smaller bubbles are achieved at the target temperature. Because the bubble number density and size can be controlled by the rapid thermal response that is characteristic of microwaves, this method could be useful for preventing superheating and bumping during nano-particle synthesis.

Special issue of the 3rd Iranian conference on heat & mass transfer

Editorial on the special issue of Heat and Mass Transfer (Springer) after the 3 rd Iranian Conference on Heat and Mass Transfer

Effect of thermo-mechanical non-equilibrium on the onset of transition in supersonic boundary layers Abstract Direct numerical simulations (DNS) for supersonic boundary layers (SBLs) with a free-stream Mach number of M∞ = 2.2 are carried out. Various cases are investigated, involving the adiabatic and the isothermal (cooled and heated) walls. The laminar boundary layer is tripped using a blowing and suction strip with single-frequency and multiple spanwise wave-number excitation. Effects of thermo-mechanical non-equilibrium of thermal boundary layer on laminar-to-turbulent transition (LTT) are presented in detail. Cases with two perturbation intensities are investigated (0.5% and 2.4%). The receptivity analysis of transition onset location towards the thermo-mechanical non-equilibrium is performed using different physical quantities like streamwise evolution of skin-friction coefficient, Stanton number and Dynamic mode decomposition (DMD). The results reveal that thermo-mechanical non-equilibrium tends to advance the transition onset location and also decreases the transition length for the heated walls regardless of the initial perturbation intensity. However, for the cooled walls with 2.4% perturbation intensity, the existence of thermo-mechanical non-equilibrium has a stabilizing effect resulting in delayed transition onset. The flow stays laminar for cooled walls with 0.5% perturbation intensity. The results obtained from DMD analysis uncover two distinct ways of evolution for odd and even harmonics of the perturbation frequency. DMD results also show that the fundamental evolution of the modes is not affected by the physical flow parameters like wall temperature or existence of thermo-mechanical non-equilibrium. It is observed that the imposed frequency mode or the principal mode is dominant in the transition region and eventually breakdown to smaller structures in the turbulent regime.

A 3-D numerical simulation of non-Newtonian blood flow through femoral artery bifurcation with a moderate arteriosclerosis: investigating Newtonian/non-Newtonian flow and its effects on elastic vessel walls Abstract In this study, a fluid-structure interaction (FSI) simulation of the blood flow in the femoral artery with a small occlusion is presented. For a more accurate simulation of the real conditions, computerized tomography (CT) scan was used to obtain a 3-D model of leg blood vessels, while the vessel was modeled as an isotropic elastic wall. By assuming a heartbeat period of 0.5 s, the inlet condition was considered as a time-dependent pulse using a non-Newtonian flow model. Blood flow was assumed nonlinear and incompressible, and Carreau model was used for blood rheological model. By considering unstable blood flow at the inlet, the involved hemodynamic parameters are velocity profile, vortices shapes, pressure drop, and streamlines. Furthermore, to determine the relationship between flow geometry and the vascular wall, wall shear stress (WSS) was calculated. By taking the real geometry of the vessel and fluidity of blood into account, comparison of computational results indicated a significant difference in velocity distribution and shear stress depending on whether the fluid-structure interaction is considered Newtonian or non-Newtonian. The results showed that employing Newtonian models for the blood flow does not lead to promising results at occluded areas and beyond them.

Simulation study on CO 2 diffusion and adsorption in zeolitic imidazolate framework-8 and -90: influence of different functional groups Abstract Global climate change or global warming due to carbon dioxide (CO2) emissions from humanity activities are believed to be one of the biggest challenges in twenty-first century, where more than 30 billion tons of this gas are released to the atmosphere annually. Among many technical procedures which can be employed to carbon capture from the flue gas stream after combustion, adsorption process utilizing advanced adsorbent materials such as highly porous Metal-Organic Frameworks (MOFs) has gained many attentions thank to many techno-economical and environmental benefits. In this paper, to enhance diffusion and adsorption capacity of CO2 inside two types of the most stable zeolitic imidazolate frameworks namely the ZIF-8 and ZIF-90, subset of MOF materials, ligand functionalization using various functional groups is accomplished through Density functional theory (DFT) and Grand Canonical Monte Carlo (GCMC) simulation. To this purpose, at first the structure of four hypothetical functionalized-ZIF-8 and -90 by NO2 (nitro group), NH2 (amino group), OH (hydroxyl group) and Li is designed using DFT calculations. Afterwards, CO2 adsorption capacity and diffusion in all functionalized-ZIFs are evaluated by GCMC simulation and molecular dynamic calculation, respectively. The accuracy of the calculated results is also evaluated by comparison with experimental data in literatures. Obtained results from simulation study revealed that in both ZIF-8 and -90 structures, Li functional group has the highest influence on the adsorption capacity for CO2 owing to high polarity of Li cations. It is believed that lithium ions create very strong negative charge centers at oxygen atoms that attack to the positive center of carbon in the CO2 molecule during adsorption.

Experimental investigation of effective factors of pulsed electric field in osmotic dehydration of apple Abstract The effects of various factors of the pulsed electric field (PEF) as an apple osmotic dehydration (OD) preprocess by using response surface methodology (RSM) are investigated and optimized in this paper. Electric field strength, pulse numbers and pulse duration were considered as PEF pretreatment effective factors in constant osmotic dehydration conditions such as concentration and temperature. After 4 h of osmosis, the results indicate decrease in amounts of water loss (WL) and solid gain (SG) by increasing field strength from 1 to 2 kV/cm and decreasing pulse duration from 0.5 to 0.02 s, respectively. The effect of increasing the pulse numbers from 8 to 16 give rise to the increase in amounts of water loss and solid gain at first and then by increasing to 24 pulses, the decrease in these amount has been evident. The ratio of water loss to solid gain (WL/SG) and response surface methodology (RSM) were used for optimization effects of PEF factors on the OD process. In optimal values of electric field strength, pulse numbers and pulse duration, the WL/SG ratio provided highest reduction sample mass.

An experimental and numerical study of the effects of reformer gas (H 2 and CO) enrichment on the natural gas homogeneous charge compression ignition (HCCI) engine Abstract Different approaches have been proposed to improve the combustion process and decrease pollutants in internal combustion engines. Among them, more attention has been paid to the use of the homogeneous charge compression ignition (HCCI) concept. The benefits of this combustion concept are the simultaneous reduction of fuel consumption, nitrogen oxides (NOx) and soot emission together with thermal efficiency improvement. However, there are still tough challenges in the successful operating of HCCI engines, such as controlling the combustion phasing, extending the operating rang, and high unburned hydrocarbon and CO emissions. In this study, experimental work were performed on a single cylinder engine and the simulation results were compared to the experimental data. The comparison showed the numerical data had a good agreement with experimental data. By increasing the ratio of reformer gas, the maximum pressure and maximum pressure rise rate are significantly increased. Keeping the air-fuel ratio constant and increasing the ratio of the reformer gas reduce the IMEP and heat efficiency, also increasing reformer gas blend fraction decreased HC and increased CO emissions considerably. In addition, using reformer gas expands the operating range lean limit and increases the possibility of engine well operation in this region.